In recent years, there has been a demand to increase the storage capacity and to reduce the size of magnetic disk drives used as an external recording devices for information processing devices such as computers. To meet the demand, the recording density of the magnetic disk drive has been increased. Accompanying with the increase in the recording density, there is an urgent demand to develop a perpendicular magnetic recording technique to increase the recording density in place of conventional longitudinal magnetic recording techniques.
In the case where a signal is written in a perpendicular magnetic recording medium using a perpendicular magnetic recording head, an electrical signal is converted into a magnetic signal by means of a coil and a magnetic flux is excited in main and auxiliary poles. The magnetic flux propagates from the auxiliary pole to the main pole. The magnetic flux then passes through a perpendicular recording layer and a soft magnetic layer formed under the perpendicular recording layer. The magnetic flux then returns to the auxiliary pole. This forms a closed circuit. In this case, the auxiliary pole is used to return, to the main pole in a magnetically efficient manner, the magnetic flux which passes through the main pole, the perpendicular recording layer and the soft magnetic layer. The signal is written as magnetization in the perpendicular recording medium using such a flow of the magnetic flux.
In perpendicular recording, it is necessary that the width of each magnetization transition region be reduced more to ensure that signals are efficiently written in regions, which are adjacent to each other and have respective magnetization directions opposite to each other. Japanese Patent Publication No. 2004-310968 (“Patent Document 1”) discloses a perpendicular magnetic recording head having a soft magnetic film mounted on the side of a main pole in a direction of a track width of a recording medium. Japanese Patent Publication No. 2005-18851 (“Patent Document 2”) discloses a perpendicular magnetic recording head capable of applying a magnetic field having a steep gradient to a recording medium due to a soft magnetic film provided on the trailing side of a main pole. Japanese Patent Publication No. 2007-35082 (“Patent Document 3”) discloses a structure in which magnetic shield films surrounding a main pole are provided. One of the magnetic shield films is provided on the main pole side to prevent a magnetic field from leaking to an adjacent track of a recording medium, and the other one is provided on the upper side (trailing side) of the main pole to cause the magnetic field to have a steep gradient.
Each of the magnetic shield films described in Patent Document 3 includes a Fe—Ni based material such as permalloy to absorb a magnetic field leaking from the main pole. A yoke is provided in contact with an edge portion of the main pole, which is located on the opposite side of the recording medium. The yoke also includes a Fe—Ni based material, such as permalloy, to cause a magnetic field to be introduced into the main pole.
Japanese Patent Publication No. 1999-161920 (“Patent Document 4”) does not disclose a perpendicular magnetic recording head, but discloses that a lower magnetic core, which also serves as an upper shield, is composed of a multi-layer film including NiFeCo films and permalloy film formed between the NiFeCo films in order to stabilize the magnetic domain structure of the lower magnetic core.
Based on the abovementioned conventional techniques, a soft magnetic film is provided around a main pole to absorb a magnetic field leaking from the main pole side to an adjacent track and to allow the magnetic field on the upper side (trailing side) of the main pole to have a steep gradient. This structure allows a signal to be written in a track having a small width.
However, when the soft magnetic films described in the conventional techniques have large crystal grains, the magnetic domain tends to have magnetization aligned in the direction of the thickness thereof. The effect to absorb a magnetic field leaking from the main pole is therefore reduced. Especially, when the soft magnetic film surrounds the main pole, the effect is significantly reduced. This makes it impossible to write a signal in a track having a small.
In order to generate a recording magnetic field having a high intensity in the main pole, it is effective that a material used for the yoke has a high saturation flux density. In addition, it is desirable that the film has high magnetic permeability to allow a high frequency signal to be written. The permalloy film has high magnetic permeability of 3000 and a low saturation flux density of 1.0 T. A film containing Ni with a weight percent of 45 and Fe with a weight percent of 55 has a high saturation flux density of 1.6 T to 1.7 T and low magnetic permeability of 1500. The film containing Ni with a weight percent of 45 and Fe with a weight percent of 55 has magnetic permeability higher than that of the permalloy film and the saturation flux density lower than that of permalloy film. To write a signal in a track having a small width, it is necessary that the material have a high saturation flux density and high magnetic permeability.
As a data transmission rate and a recording density of a magnetic disk drive are increased, a frequency of a signal to be written is increased. A perpendicular recording technique may be adopted to write such a high frequency signal. However, such a technique may have a problem in that a signal is erroneously deleted after being written.